The present invention relates generally to the use of a degradable solution of any kind of product, and more particularly through not exclusively, to the use of a saturable absorber in a laser for locked mode operation.
In some applications such as an ophthalmic surgery laser apparatus it is desirable, or even necessary, for the pulses of the laser beam to be extremely short, of the order of a maximum of several tens of picoseconds so that the energy of the light flux transported by the laser beam is sufficient to produce the sought-after effects.
As is known, and without it being necessary to give greater details in this respect in the present application, the laser beam emitted is controlled in a suitable cavity by a control device commonly known as a Q switch which permits emergence of the laser beam only when such very short duration pulses are produced.
By way of example of such a Q switch it is known to use in such a cavity, which is in practice defined by a cell, a solution of a saturable absorber the chemical composition of which is adapted to change abruptly and irreversibly, and therefore also its physical properties, under the effects of such a laser beam.
In practice such a saturable absorber is first opaque to laser radiation and then becomes abruptly transparent, and vice versa. The product sold by Kodak under reference 9740 and commonly referred to as DYE which is used in a chlorobenzene solution has proved to be satisfactory.
Unfortunately, in actual use such a product has been found to be degradable to the extent that under the effects of the light radiation, and particularly the ultraviolet composant thereof, and also the oxygen of the ambiant atmosphere, it gradually loses operating capacity. The operation of the laser equipped with a DYE cell uncontrollably changes from the locked mode to the Q switching mode which is detrimental to the sought-after results in at least some fields of use.
To at least partially overcome this drawback, and taking into account the fact that in practice only a small area of the DYE cell, herein referred to as the active zone, is in fact affected by the laser beam, it has already been proposed to put such a DYE cell in a circulating circuit having a series connected pump for pumping the solution and thereby continuously changing the part of the solution present in active zone of the cell.
But when all the solution in the circulation system has degraded below a threshold level the entire solution must be changed. This is a relatively time-consuming and delicate operation during which period the laser apparatus may not be used and if special monitoring means are not employed the solution may not be changed as soon as actually necessary.
It is also known in U.S. Pat. No. 2,978,951 and French Pat. No. 427,128, which deal with a solution of one of more conventional dyes and not with a degradable solution within the meaning of the present invention, to connect to a circuit using such a solution or a circuit for circulating the solution, at least one tank containing at least one dye constituent a predetermined amount of which is injected, at will, under the control of valves, into the circuit to readjust the solution.
Such a readjustment of the dye solution is not necessarily suitable for degradable solutions of the type concerned by the present invention, namely in case all the resulting products of degradation cannot be allowed to remain in the solution in the circulation circuit without causing operating problems and/or in case the circulation circuit contains a fixed amount of solution.